Texas A&M University, Department of Civil Engineering

1
Processing Geospatial Data with HEC-GeoRAS 3.1
Texas AM University, Department of Civil
Engineering CVEN 689 Applications of GIS in Civil
Engineering Professor Dr. Francisco
Olivera Student Brad W. Endres 28 April 2003
Aerial photo of Bolinas Lagoon, CA. USACE.
Demonstration of Capabilities
Extension, users manual, and example data are
available at www.hec.usace.army.mil
Abstract The use of geospatial information is
increasingly useful in hydrologic and hydraulic
modeling through computer simulation. Geographic
Information Systems (GIS) software such as
ArcView 3.2 and the Hydrologic Engineering
Centers River Analysis System (HEC-RAS 3.1) are
software programs that utilize digital spatial
and temporal information to model the flow of
water on the earths surface. HEC-GeoRAS 3.1
extends the capabilities of ArcView by extracting
and formatting hydraulic parameters that are
required to model streams and rivers in HEC-RAS.
As an interface between two distinct software
programs, GeoRAS makes it possible to utilize
digital data files where calculations and
drawings were conventionally done by hand. By
employing these digital tools, engineers not only
increase the calculation accuracy, but they
increase the number of scenarios that can be
quickly modeled through computer processing.
Initially, an investment of time is required to
master these software programs, but the savings
in time and money associated with detailed
hydraulic analysis is worth the effort.
This depiction of the ArcView GIS 3.2 menu shows
the data processing (drop down) menus of the
HEC-GeoRAS extension.
PreRAS
After loading the 3D Analyst, Spatial Analyst,
and the GeoRAS extensions, the user can add TIN
data to begin the project. The features of the
Pre-RAS menu allow the user to generate the
geometric data and the RAS GIS import file that
will transport the data in a readable format for
HEC-RAS. The items in the drop down menu should
be followed in order to ensure accurate
completion of the geometric data. Using the
drawing tools of ArcView 3.2, the user makes line
themes in the project. Engineering judgment is
applied when drawing the center of the stream,
its banks, and the over bank flow paths.
3-D scene of the Wailupe River TIN. Triangular
Irregular Networks provide precise elevation data
needed for channel bottoms.
3-D scene of the contours of the Wailupe River
TIN. Using the contour theme allows for faster
display of the background layer.
This image represents the line themes that the
user creates under the Pre-RAS menu. Using basic
ArcView drawing tools, the user creates the
Stream Centerline, Main Channel Banks, Flow Path
Centerlines, Cross Section Cut Lines, and adds
polygon themes as well.
HEC-RAS
Also free at the HEC website, HEC-RAS performs
one-dimensional steady and unsteady flow
calculations for a natural or constructed
channel. HEC-GeoRAS allows the transfer of
geometric data from ArcView to be used in the
Geometric Data Editor of HEC-RAS. The user
completes any additional geometry, provides flow
data, provides boundary conditions, and performs
a steady flow analysis (in this case). The
resulting calculations from solving the energy
equation indicate the water surface profile and
other data in tabular or graphical form.
This image from the Geometric Data Editor of
HEC-RAS indicates the successful transfer of the
RAS GIS import file of Wailupe River.
After entering the streams geometric data and
flow data, HEC-RAS computes the water surface
profile at each cross section.
HEC-RAS computes water surface profiles
throughout the reach for steady gradually varied
flow (in this example). The X-Y-Z plot under the
View menu illustrates the extent to which the
flood event exceeds the river banks. This data
is exported to ArcView.
PostRAS
PostRAS
Following satisfactory results from HEC-RAS, the
RAS GIS export file is loaded into the ArcView
project using the commands under the PostRAS drop
down menu. Floodplain delineation, water depth,
and water velocity themes may be generated using
the PostRAS features. The base themes that are
created include the stream network, cross
sectional cut lines, cross section surface lines,
bank station lines, water surface profile
bounding polygons, and velocity mass points. A
water surface TIN is created independent of the
terrain TIN. Next, the water surface TIN and the
terrain TIN are rasterized to create the
floodplain areas where the water surface
elevations are higher than the terrain
elevations.
ArcView makes additional grid and TIN data from
the previously rasterized water surface TIN and
terrain TIN. When subtracted, a depth TIN
results. The user can select the identify tool
and click on the depth grid to obtain a discrete
value. If the velocity data was included in the
RAS GIS export file, a velocity TIN can be
generated using the velocity mass point
shapefile. The values are interpolated between
the points, the user must consider this when
viewing results. Next, a velocity grid can be
rasterized from the velocity TIN. Again,
discrete values are interpolated between the
cross sections.
3-D scene of the Wailupe contours with the
delineated floodplain.
The velocity grid is created. Dark blue
indicates faster flowing water.
The depth grid is created. Dark blue indicates
deeper water.
Application to Study Area
Bolinas Lagoon, CA
Analysis with GeoRAS
Results
In comparison with a simulation using 1968 data,
the main channel of the lagoon appears to be
getting deeper while the overall elevation of the
lagoon is rising. This result is consistent with
the notion that watershed runoff is entering the
lagoon at a higher velocity and with a higher
sediment load. The GIS analysis suggests that
the main channel within the lagoon is
experiencing scour due to the higher runoff
velocity. The deeper channel may contribute to
the measured decrease in the tidal prism for the
lagoon. Although this application should be
validated with further study, it demonstrates the
engineering utility and capability of GIS tools.
Bolinas Lagoon, California is an environmentally
sensitive estuarine habitat located 15 miles
north of San Francisco, CA. Development within
the watershed of the lagoon has caused
accelerated sedimentation which is slowly
destroying aquatic life and disrupting the
habitat of seals and birds. The lagoons surface
area is 1100 acres and consists of tidal
channels, mudflats, and marshes. The watershed
is 16.7 square miles of steep ridges and small
streams with high velocities. USACE has studied
several restoration plans.
Using a TIN of the 1988 conditions of lagoon and
known water surface elevations, the line themes
and land use themes were created for import to
HEC-RAS for analysis of the main channel within
the lagoon. HEC-RAS computes the water surface
profile at each cross section, and flow and
velocity distribution data if selected.
HEC-GeoRAS allows this information to be read
into ArcView for the generation of the water
surface TIN and floodplain themes. A depth grid
can be created to show the discrete value of
water surface elevation above the channel bottom.
These GIS tools are effective in delineating a
floodplain for a given flow condition, as well as
providing depth and velocity grids for further
engineering analysis.